1. Field of the Invention
The present invention relates to a foamed polymer and to a process for production thereof. More particularly, the present invention relates to a foamed isocyanate-based polymer (e.g. polyurethane foam, polyurea foam, polyisocyanurate foam, etc.) and a process for production thereof.
2. Description of the Prior Art
Isocyanate-based polymers are known in the art. Generally, those of skill in the art understand isocyanate-based polymers to be polyurethanes, polyureas, polyisocyanurates and mixtures thereof.
It is also known in the art to produce foamed isocyanate-based polymers. Indeed, one of the advantages of isocyanate-based polymers compared to other polymer systems is that the chemistry can be used to achieve desired product properties in situ.
One of the conventional ways to produce a polyurethane foam is known as the "one-shot" technique. In this technique, the isocyanate, a suitable polyol, a catalyst, water (which acts to generate carbon dioxide as the blowing agent and can optionally be supplemented with one or more secondary organic blowing agents) and other additives are mixed together at once using, for example, a mechanical or impingement mixer. Generally, if one were to produce a polyurea, the polyol would be replaced with a suitable polyamine. A polyisocyanurate may result from cyclotrimerization of the isocyanate component. Urethane-modified polyureas or polyisocyanurates are known in the art. In either scenario, the reactants would be intimately mixed quickly using a suitable mixer.
Another technique for producing foamed isocyanate-based polymers is known as the "prepolymer" technique. In this technique, a prepolymer of polyol and isocyanate (in the case of a polyurethane) are reacted in an inert atmosphere to form a liquid polymer terminated with isocyanate groups. To produce the foamed polymer, the prepolymer is thoroughly mixed with water and, optionally, a polyol (in the case of producing a polyurethane) or a polyamine (in the case of producing a polyurea) in the presence of a catalyst or a cross-linker.
As is known by those of skill in the art, many conventional isocyanate-based foams are non-hydrophilic (i.e. relatively hydrophobic). Such foams generally have an aversion to aqueous fluids. Practically, this results in such foams being unable to absorb or pick up significant quantities of aqueous fluids (e.g. the foams will float on water) other than by mechanical entrainment. Accordingly, such foams are vimally never used in an application in which significant aqueous fluid absorption and retention is a desired feature.
In copending U.S. patent application Ser. Nos. 08/413,433 (Wilson) and 08/554,896 (Wilson), the contents of each of which are hereby incorporated by reference, there is disclosed a foamed isocyanate-based polymer having a cellular structure and containing a superabsorbent material, the polymer being capable of: (i) absorbing at least about 20 times its weight of an aqueous fluid maintained at a temperature of from about 20.degree. to about 25.degree. C. and (ii) retaining at least about 20 times its weight of absorbed aqueous fluid which is bound to the superabsorbent material. The foamed isocyanate-based polymer disclosed in the Wilson '433 and '896 applications may be produced by a process which comprises reacting and expanding (via a suitable catalyst and blowing agent) a mixture comprising an isocyanate, an active hydrogen-containing compound and a superabsorbent material. The superabsorbent material is capable of absorbing at least about 20 times its weight of an aqueous fluid maintained at a temperature in the range of from about 20.degree. to about 25.degree. C. The active hydrogen-containing compound comprises from about 10% to 100% by weight of a hydrophilic active hydrogen-containing compound and from 0 to about 90% by weight a non-hydrophilic active hydrogen-containing compound. The foamed isocyanate-based polymer is ideally suitable for use in an absorption layer in a personal hygiene device.
An example of known superabsorbent materials are superabsorbent polymers. A good discussion on superabsorbent polymers may be found in "SUPERABSORBENT POLYMERS Science and Technology", ACS Symposium Series 573, Edited by Bucholz et al. (1994), the contents of which are hereby incorporated by reference.
A superabsorbent material, such as a superabsorbent polymer, may be thought of as an ionic hydrocolloid. Generally, such a material is considered to be superabsorbent if it is able to imbibe, absorb or gel at least about 10 times its weight of a fluid and retain the fluid under moderate pressure (this property is also known as Absorbency Under Load or AUL. and is discussed in more detail hereinbelow), for example using the protocol discussed U.S. Pat. No. 5,147,343, the contents of which are hereby incorporated by reference.
Current conventional, commercial superabsorbent polymers are cross-linked polymers of partially neutralized acrylic acid--see Chapter 2 of "SUPERABSORBENT POLYMERS Science and Technology", ACS Symposium Series 573, Edited by Bucholz et al. (1994), the contents of which are hereby incorporated by reference. The crosslinked polymers are actually terpolymers of acrylic acid, sodium acrylate and a crosslinker. Such polymers may be produced by free-radical polymerization in aqueous solution, graft copolymerization or suspension polymerization and, depending on the degree of cross-linking of the polymers, are referred to as first generation (lightly cross-linked) and second generation (cross-linked) superabsorbent polymers.
Once the cross-linked polymers are produced (these may be regarded as "non-surface cross-linked superabsorbent polymers"), it is known in the art to post-treat them to effect surface cross-linking (these post-treated materials may be regarded as "surface cross-linked superabsorbent polymers"). This further cross-linking at the surface of the particles is known to alter the absorption rate of the polymer--see published British patent application 2,119,384 and U.S. Pat. No. 4,497,930, the contents of each of which are hereby incorporated by reference.
Surface cross-linked superabsorbent polymers contain a discontinuous network and thus, have offered a number of variations in product properties. As discussed in Chapter 8 of "SUPERABSORBENT POLYMERS Science and Technology", ACS Symposium Series 573, Edited by Bucholz et al. (1994), the contents of which are hereby incorporated by reference, first and second generation (non-surface cross-linked) superabsorbent polymers suffer from the fact that their retention capacity varies directly with the degree of cross-linking whereas their AUL varies indirectly with the degree of cross-linking--this is illustrated in FIG. 1 which includes retention capacity and AUL for first and second generation superabsorbent polymers. Third generation (surface cross-linked) superabsorbent polymers were developed as an improvement over first and second generation superabsorbent polymers--this is illustrated in FIG. 2 which includes retention capacity and AUL for third generation superabsorbent polymers. Thus, third generation (surface cross-linked) superabsorbent polymers are characterized by an increased AUL compared to second generation (non-surface cross-linked) superabsorbent polymers. Not surprisingly, third generation (surface cross-linked) superabsorbent polymers are significantly more costly (e.g. 10% or more) than second generation (nonsurface cross-linked) superabsorbent polymers. Thus, on an equivalent basis, improved performance is achieved at an increased cost.
While the foamed isocyanate-based polymer disclosed in the Wilson '433 and '896 applications represents a significant advance in the art, there is continuous need for improvements in the art. For example, there is a continuous need to improve and optimize the fluid absorbing efficiency of the superabsorbent material contained in the foam matrix. Depending on the particular application for the foamed polymer, such an improvement would allow one or more of: (i) reduction of the amount of superabsorbent material required to meet a specified fluid absorption/retention (thereby reducing the cost of producing the foamed polymer), (ii) the ability to improve the performance of the foamed polymer (i.e. increasing the fluid absorption/retention thereof) beyond that conventionally obtained, and (iii) reduction in the cost of product of the foamed polymer to meet a specific performance criterion.
In light of the above, it would be advantageous to have a foamed isocyanate-based polymer which is hydrophilic and characterized by improved absorption (or pick up) and retention of an aqueous fluid. It would be further advantageous if such a foam could be produced in a relatively uncomplicated way and possessed reproducible physical properties.